Trigger transformer for pulse forming network

ABSTRACT

The inductor of a pulse forming network comprising a storage capacitor and inductor is provided by the secondary winding of a series-connected trigger transformer. Electromagnetic radiation is minimized while the stored energy is discharged through a flash lamp with a current waveform determined by the inductance of the secondary winding and the capacitance of the storage capacitor. The waveform is maintained at a desired level determined by the design of the secondary windings on cores of high saturation material. The secondary windings are tightly wound to minimize radiation from the sides of the cores, and the ends of the cores are closed by high permeability, high saturation density material to minimize radiation of the ends of the cores. The cap which prevents saturation of the main transformer core is filled with high permeability, low saturation density material to facilitate use as a trigger transformer.

United States Patent Dahlinger et al.

[54] TRIGGER TRANSFORMER FOR PULSE FORMING NETWORK [72] Inventors: Rodney J. Dahlinger, Canoga Park; Robert P. Farnsworth, Los Angeles,

both of Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: Aug. 12, 1970 [21 Appl. N6; 63,136

Hockenberry et al .336/178 X Oct. 24, 1972 2,886,790 5/1959 Snyder..... ....336/212X 3,094,626 6/1963 Brewster ..336/212X Primary Examiner-Robert K. Schaefer Assistant Examiner-William J. Smith Attorney-James K. Haskell and Lawrence V. Link, Jr.

[57] ABSTRACT The inductor of a pulse forming network comprising a storage capacitor and inductor is provided by the secondary winding of a series-connected trigger transformer. Electromagnetic radiation is minimized while the stored energy is discharged through a flash lamp with a current waveform determined by the inductance of the secondary winding and the capacitance of the storage capacitor. The wavefonn is maintained at a desired level determined by the design of the secondary windings on cores of high saturation material. The secondary windings are tightly wound to minimize radiation from the sides of the cores, and the ends of the cores are closed by high permeability, high saturation density material to minimize radiation of the ends of the cores. The cap which prevents saturation of the main transformer core is filled with high permeability, low saturation density material to facilitate use as a trigger transformer.

10 Claims, 2 Drawing Figures TRIGGER TRANSFORMER FOR PULSE FORMING 7' NETWORK BACKGROUND OF THE INVENTION This invention relates to a circuit for triggering a load in the discharge circuit of a pulse forming network (PFN), and more particularly to apparatus for combining the secondary winding of a trigger transformer with a pulse shaping coil in the output section of the PFN in order to minimize radiation of electromagnetic inter ference (EMI).

In the past, a trigger transformer has been separately provided to cause a PFN to discharge through a load. The arrangements have used both series and shunt triggering. In a shunt triggering arrangement, the secondary winding of the trigger transformer is connected to a trigger electrode of the load, which may be a flash lamp, a thyratron coupled in the primary circuit of a step-up transformer for a magnetron; or the like. When a pulse is applied to the primary winding of the trigger transformer, the high potential across the secondary winding starts the initial ionization of the flash lamp and the stored charge of the PFN maintains the ionization as the PFN discharges.

When the sole purpose of the PFN is to store energy, a simple capacitor will do, but in many applications, such as in lasers, the PFN must store energy and shape the output waveform to furnish a steady output current for a certain length of time, at which time its voltage may drop to zero. To accomplish current control, series inductance must be included in the PFN. Although good triggering and pulse shaping is achieved by such a shunt arrangement, the EMI of the prior art PFN inductance adds to the EMI of the trigger transformer.

In a series triggering arrangement, the secondary of the triggering transfonn'er is connected in series between the PFN and the load. A trigger electrode is not employed to start the initial ionization. Instead, the potential induced across the secondary is added to the potential of the stored energy to initiate ionization. The PFN then discharges as in the case of the shunt arrangement with EMI from PFN inductance adding to EMI from the trigger transformer.

SUMMARY OF THE INVENTION An object of this invention is to minimize radiation of electromagnetic interference from pulse forming network.

Another object is to combine into a pulse forming network a pulse forming trigger transformer.

This and other objects of the present invention are achieved by replacing the inductance coil of a pulse forming network with the secondary winding of a trigger transformer in a series arrangement for triggering the load connected to a PFN. By designing the series connected trigger transformer to have high inductance during initial triggering and low inductance during discharge of the stored energy following the initial pulse, the secondary winding of the transformer may be used as the inductance coil of the pulse forming network. That is accomplished by making the inductance of the transformer equal to the desired pulse forming inductance of a series output coil for current flow in the secondary winding above a predetermined level required for triggering initial discharge through the load by using suitably low saturation density material for the core gap or gaps. The secondary winding completely envelopes the low saturation gap material of the trigger transformer to minimize radiation of electromagnetic interference from the gap or gaps. High saturation density material is used to provide low impedance for return flux from one end of the secondary winding to the other.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates schematically a preferred embodiment of the present invention.

FIG. 2 is a diagram illustrating the characteristics of a transformer in the embodiment of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a storage capacitor 10 is charged from a power supply (not shown) through a charging circuit 11 and a diode D The storage capacitor 10 is connected to a flash lamp 12 for a laser (not shown) located in an enclosing cavity 13.

The capacitor 10 is connected to the flash lamp 12 by a triggering transformer designated generally by the reference numeral 14. It has two parallel secondary windings S, and S connected in series between the capacitor 10 and one end of the flash lamp 12. Two primary windings P and P are connected in series between a pulse source 15 and ground.

The core of the transformer. 14 is shown in section along with the primary and secondary windings to facilitate illustrating the present invention which consists of using the inductance of the trigger transformer 14 for current waveform shaping by proper design of the inductance of the secondary windings S, and S The design employs a core having two sections 16 and 17 of high saturation material, such as silicon steel, joined by sections 18 and 19 of low saturation material, such as a ferrite.

When the transformer 14 is pulsed to trigger the discharge of energy stored in the capacitor 10 through the flash lamp 12, the gas in the lamp 12 is ionized and current will begin to flow from the capacitor 10. Thereafter, the potential of the energy stored in the capacitor 10 will maintain ionization of the gas in the lamp 12 until the stored energy has been discharged to a level which is insufficient to maintain ionization. To insure the proper current waveform, the inductance of the secondary winding of the trigger transformer must be decreased after initial ionization of the lamp 12 to the proper value for the discharge pulse forming function.

To achieve the desired triggering waveform, the transformer is designed with a first inductance in the secondary winding of a value represented by the slope of a curve in the graph of FIG. 2 for a current amplitudes from zero to I, for use during the initial (pulsed) period and with a second inductance represented by the slope of the curve from I, to l -for use during the ensuing PFN discharge period.

The characteristic curve of the secondary winding inductance for the transformer 14 from zero to I, is provided by the high saturation density characteristics of the material" used for the basic core (ends 16 and 17 and the low saturation density characteristics of the material used for the center sections 18 and 19 within the closely wound secondary windings S and S Once the transformer 14 has been triggered by a pulse from the source 15, and conduction has increased suffrciently to saturate the sections 18 and 19, the entire core assembly will behave as an air-gapped core to provide the inductance represented by the slope of the curve in FIG. 2 between the current levels I and I At the current level of 1 both the low-saturation, center sections 18 and 19 and the high-saturation, end sections 16 and 17 become saturated to provide an inductance represented by the slope of the curve in FIG. 2 above the current level of I However, the choice of capacitance and transformer inductance limits the current to a level 1 below I so that the capacitor and the inductance of the secondary windings S and S, will insure the proper current waveform.

During the initial period of the discharge cycle, the current from the capacitor 10 through the secondary windings S and S is below the level I, and neither the high saturation density material of the end sections 16 and 17 nor the low saturation density material of the center sections 18 and '19 are saturated. Therefore, very little electromagnetic interference is radiated. Once the gas in the lamp 12 has been ionized, the current will rapidly increase to the maximum level I As i the current passes through the level 1 the low saturation center sections 18 and 19 become saturated to switch the inductance of the secondary windings S and S to the proper value. Since the end sections 16 and 17 are still unsaturated, very little electromagnetic interference is radiated from the ends of the saturated core sections 18 and 19. At the same time very little electromagnetic interference is radiated from the sides of the saturated core sections 18 and 19 because the secondary windings S and S are tightly wound with each turn placed close by adjacent turns. Thus, although the core sections 18 and 19 are permitted to become saturated in order to provide the proper current waveform to the load (lamp 12), there is very little electromagnetic radiation because the tightly wound secondary windings S and S confine the flux passing through the ends of the secondary windings S, and S From the foregoing it may be readily appreciated that closed loop core of high saturation density material and tightly wound secondary windings minimizes electromagnetic radiation from the secondary windings .when used as part of a pulse forming network when connected in series between the storage capacitance and the load, and that the core section of low saturation density material within the secondary windings will provide the necessary inductance for the trigger transformer. In an application of the present invention for a particular laser, the transformer was designed for a primary inductance of 121th and a secondary inductance of 4.8mh during the initial trigger pulse period using a turn ratio of l to 20.

Once the center sections 18 and 19 have become saturated, the inductance provided by the secondary windings S and S is decreased to 75uh. The material selected for the sections 18 and 19 was 337 ferrite material. However, it should be appreciated that different applications may require different designs, and that different materials and arrangements may be employed to satisfy particular design requirements without departing from the present invention. For example, the primary windings P, and P may be wound around the sections 18 and 19, instead of the end sections 16 and 17, and the end sections may be omitted if the center sections are, for example, formed as one toroid unit and the secondary windings are so wound that the ends of one abut the ends of the other to confine the magnetic flux to the toroid core of the low saturation density material after saturation has occurred.

In some applications, only one winding may be used for the primary, or the secondary, or both, and in the case of dual windings the series and parallel arrangements may be reversed from the arrangement shown, or both may be in parallel or in series. The important design consideration is that the low saturation density material of the core be saturated when the stored energy starts to discharge through the secondary winding, or windings, and the flash lamp, and that the low saturation material be completely enclosed by the tightly wound secondary winding or windings.

If the ends of the secondary winding(s) do not abut, material that never becomes saturated by the maximum discharge current allowed should be used to close the ends and provide a continuous flux path from one to the other. Thus the secondary winding(s) around the center section(s) of the core which determine the desired inductance during the flash lamp discharge are made to contain the flux field in the center section(s), while unsaturated core material contains the field outside of the secondary winding(s). This results in minimized radiation, not only because radiation has been minimized in the pulse trigger transformer, but also because the need for a separate inductance coil is abreviated.

What is claimed is:

1. In a circuit, responsive to an applied trigger pulse, for energizing a load in the discharge circuit of a pulse forming network; a transformer having a core, a primary winding disposed on said core and adapted for receiving said trigger pulse, a secondary winding coupled in a series arrangement between an energy storage circuit of said pulse forming network and said load with said secondary winding being disposed on a section of core which has saturation characteristics such that said section is saturated during the time of application of said trigger pulse and during the discharge of energy from said energy storage circuit into said load, and said secondary winding has an inductance value when said section is saturated so that the discharge of energy into said load is in accordance with a predetermined current waveform.

2. The combination of claim 1 wherein said core is closed and the remaining portion thereof, other than said section, is composed of a material having a saturation characteristic sufficiently high to prevent said remaining portion from saturating during said discharge of energy into said load; thereby reducing electromagnetic radiation.

3. The combination of claim 2 wherein said secondary winding is tightly wound around and encompasses said core section, thereby further reducing electromagnetic radiation.

4. The combination of claim 1 including flux path means for confining the flux field produced by the discharge of energy through said secondary winding; thereby reducing electromagnetic radiation.

5. The combination of claim 4 wherein said flux path means includes the remaining portion of said core other than said section, said portion being composed of a sufliciently high saturation characteristic material as to remain unsaturated when said energy discharge rate reaches its maximum level, and said remaining core portion being disposed to close off the ends of said core section and provide a continuous flux path between said ends.

6. In a circuit for triggering a load in the discharge circuit of a pulse forming network, a transformer having a secondary winding in series between a storage capacitance network of said pulse forming network and said load, and having a primary winding adapted to receive a trigger pulse, said transformer having a closed core of high permeability, high saturation material except in a section fully contained within said secondary winding, said section being filled with material of high permeability, low saturation characteristics; whereby higher inductance is provided by said secondary winding during an initial period while a trigger pulse is applied to said primary winding to trigger said load into conduction, than during an ensuing period while said storage capacitance network is discharged through said secondary winding and said load.

7. The combination of claim 6 wherein said secondary winding is tightly wound around said core section to contain flux therein.

8.- The combination of claim 7 wherein the inductance of said secondary winding during saturation of said core section is of such a predetermined value as to limit current conduction through said load to a predetermined maximum level below a saturating level for said high saturation portion of said core.

9. In a circuit for triggering a load in the discharge circuit of a pulse forming network, a trigger transformer having a secondary winding in a series arrangement for triggering said load into conduction in response to a pulse applied to a primary winding of said transformer, whereupon stored energy in said network is discharged through said load and said secondary winding, said transformer having a closed core of high permeability, high saturation density material throughout except in a section thereof completely encompassed by said secondary winding, and said section being composed of high permeability, low saturation density material; whereby the inductance of said secondary winding is more during the application of said pulse and initial period of discharge of said load for current flow below a predetermined level, than during the ensuing discharge of said energy for current flow above said predetermined level.

10. The combination of claim 9 wherein said secondary winding is tightly wound around said core section. 

1. In a circuit, responsive to an applied trigger pulse, for energizing a load in the discharge circuit of a pulse forming network; a transformer having a core, a primary winding disposed on said core and adapted for receiving said trigger pulse, a secondary winding coupled in a series arrangement between an energy storage circuit of said pulse forming network and said load with said secondary winding being disposed on a section of core which has saturation characteristics such that said section is saturated during the time of application of said trigger pulse and during the discharge of energy from said energy storage circuit into said load, and said secondary winding has an inductance value when said section is saturated so that the discharge of energy into said load is in accordance with a predetermined current waveform.
 2. The combination of claim 1 wherein said core is closed and the remaining portion thereof, other than said section, is composed of a material having a saturation characteristic sufficiently high to prevent said remaining portion from saturating during said discharge of energy into said load; thereby reducing electromagnetic radiation.
 3. The combination of claim 2 wherein said secondary winding is tightly wound around and encompasses said core section, thereby further reducing electromagnetic radiation.
 4. The combination of claim 1 including flux path means for confining the flux field produced by the discharge of energy through said secondary winding; thereby reducing electromagnetic radiation.
 5. The combination of claim 4 wherein said flux path means includes the remaining portion of said core other than said section, said portion being composed of a sufficiently high saturation characteristic material as to remain unsaturated when said energy discharge rate reaches its maximum level, and said remaining core portion being disposed to close off the ends of said core section and provide a continuous flux path between said ends.
 6. In a circuit for triggering a load in the discharge circuit of a pulse forming network, a transformer having a secondary winding in series between a storage capacitance network of said pulse forming network and said load, and having a primary winding adapted to receive a trigger pulse, said transformer having a closed core of high permeability, high saturation material except in a section fully contained within said secondary winding, said section being filled with material of high permeability, low saturation characteristics; whereby higher inductance is provided by said secondary winding during an initial period while a trigger pulse is applied to said primary winding to trigger said load into conduction, than during an ensuing period while said storage capacitance network is discharged through said secondary winding and said load.
 7. The combination of claim 6 wherein said secondary winding is tightly wound around said core section to contain flux therein.
 8. The combination of claim 7 wherein the inductance of said secondary winding during saturation of said core section is of such a predetermined value as to limit current conduction through said load to a predetermined maximum level below a saturating level for said high saturation portion of said core.
 9. In a circuit for triggering a load in the discharge circuit of a pulse forming network, a trigger transformer having a secondary winding in a series arrangement for triggering said load into conduction in response to a pulse applied to a primary winding of said transformer, whereupon stored energy in said network is discharged through said load and said secondary winding, said transformer having a closed core of high permeability, high saturation density material throughout except in a section thereof completely encompassed by said secondary winding, and said section being composed of high permeability, low saturation density material; whereby the inductance of said secondary winding is more during the application of said pulse and initial period of discharge of said load for current flow below a predetermined level, than during the ensuing discharge of said energy for current flow above said predetermined level.
 10. The combination of claim 9 wherein said secondary winding is tightly wound around said core section. 